Overall Loss Compensation and Optimization Control in Single-Stage Inductive Power Transfer Converter Delivering Constant Power

A typical battery charging process consists of a constant-current charging phase which is followed and completed by a constant-voltage charging phase. Moreover, replacing the constant-current charging by constant-power charging can eliminate thermal problems and enhance the cycle life of the battery. This work aims to maximize the system efficiency of a single stage inductive power transfer (IPT) charger by minimizing the overall losses using a constant-power charging scheme. The single-stage constant-power IPT charger employs series-series compensation and adopts an active rectifier on the secondary side. Based on a time-domain model, the conditions of zero voltage-switching (ZVS) and minimum circulating reactive power are derived. Then, the power losses in the magnetic coupler, inverter and active rectifier are analyzed and optimized under constant power output condition. Combining the conditions of ZVS, minimum circulating reactive power and minimum overall losses, we propose a novel optimal control strategy to maintain constant power output and maximum efficiency throughout the charging process. In addition, the proportional integral (PI) controller is not needed. Finally, a 120 W experimental prototype is built to verify the performance of the proposed control strategy. Experimental results demonstrate high precision constant power output and an efficiency of around 87.5% for the proposed single stage inductive power transfer battery charger.